The invention pertains to formulations employing aqueous dispersions of polyurethanes to which are added crosslinking agents at the time of use. One particularly important application of such dispersions is as adhesives in the manufacture of laminates from thin flexible sheet substrates. The aqueous dispersion is mixed with the crosslinking agent and then applied to a first sheet as a thin film. The aqueous vehicle is periled to evaporate from the adhesive film and the adhesive is then contacted with a second sheet, typically with the application of heat and/or pressure, to join the sheets into a laminate.
Large amounts of such laminates are used in the packaging industry, especially in making food packaging, and it is therefore desirable for environmental, health and safety reasons that the adhesives employed in forming the laminate and in bonding packaging prepared therefrom be solvent free. It is also important that the adhesive bonds be capable of withstanding high temperatures and humidities, for instance boiling water exposure.
Kucera et al., U.S. Pat. No. 4,540,633, discloses polyurethane dispersion adhesive compositions formed by mixing a polyurethane dispersion and an epoxy or polyisocyanate.
Ramalingam, U.S. Pat. No. 4,883,694, discloses water dispersed polyurethane adhesives for flexible film laminates in which peroxide extended polyurethane dispersions are crosslinked using an epoxy functional compound.
Scriven et al., U.S. Pat. No. 4,147,679, discloses thermosettable polyurethane dispersion coating formulations, mentioning as curing agents aminoplast resins, formaldehyde, phenolic resins, alkoxysilanes, organic polyisoeyanates (blocked or free) and epoxies.
Holmbach et al., U.S. Pat. Nos. 4,433,095 and 4,663,377, respectively disclose aromatic and aliphatic polyisocyanate dispersable compositions useful as crosslinking agents for polyurethane dispersion adhesives.
Wolfrey, U.S. Pat. No. 4,301,053, describes aqueous polyurethane coating dispersion formulations employing aziridine compound crosslinkers. The coatings, however, are described as hard and in order to obtain adherence to non-polar thermoplastic substrates it is said to be necessary to add N-methylpyrrolidone solvent to the formulation.